This invention relates to the charging/discharging control of lithium-ion batteries, and, more particularly, to an approach that protects the batteries and optimizes their performance.
Spacecraft such as geosynchronous communications satellites consume large amounts of power in the amplifiers and other electronics that relay signals from point to point on the earth's surface. The power is normally generated by solar cells which produce power only when sunlight is incident upon the solar cells and do not generate power when the satellite is in the earth's shadow. A battery system with rechargeable cells is therefore provided onboard the spacecraft to receive and store excess power generated by the solar cells when the spacecraft is in sunlight, and to deliver that power to the power-consuming components when the spacecraft is in shadow.
Rechargeable cells or batteries are electrochemical energy storage devices for storing and retaining an electrical charge and later delivering that charge as useful power, and which may then be recharged. Familiar examples of the rechargeable energy storage cell are the lead-acid cell used in automobiles, and the nickel-cadmium cell used in various portable electronic devices. The weight of the spacecraft energy storage cell must be minimized while achieving the required performance level, due to the cost of lifting weight to earth orbit and beyond. A battery having even greater performance per unit weight is the lithium-ion battery.
A typical lithium ion battery cell includes a negative electrode, a positive electrode, a separator between the negative electrode and the positive electrode, an electrolyte that saturates the separator and provides a lithium ion path between the negative electrode and the positive electrode, a negative current collector contacting the negative electrode, and a positive current collector contacting the positive electrode. The negative electrode includes a negative-electrode active material that releases lithium ions upon discharging of the battery cell and absorbs lithium ions upon charging of the battery cell. The positive electrode includes a positive-electrode active material that reacts with lithium ions upon discharging of the battery cell and releases lithium ions upon charging of the battery cell. A lithium-ion battery typically employs a number of individual battery cells electrically interconnected in series or parallel arrangements to deliver the required voltage and current for the spacecraft.
Although the rechargeable lithium-ion battery has a great capacity to store electrical charge, it is relatively easily damaged in an electrical sense. For example, overcharging the battery may lead to oxidation and decomposition of the electrolyte, and over-discharging of the battery may cause a change in the physical structure of the negative cathode. Recognizing this problem, a number of battery controllers and controller strategies have been developed to implement the charging and discharging of the lithium-ion batteries. While operable to some degree, these existing approaches require large numbers of components that add considerable weight to the spacecraft. They do not achieve an optimum charging and discharging cycle, and are lacking in some elements of control.
There is a need for an improved approach to the controlling of lithium-ion batteries, particularly those intended for spacecraft applications where the weight of the system must be minimized. The present invention fulfills this need, and further provides related advantages.